The present invention is generally related to electronic circuitry, and more particularly to Printed Circuit Boards (PCBs).
Electronic systems often comprise a motherboard coupled to one or more daughtercards, with the motherboard providing the electrical interconnections between the daughtercards. In aircraft communication systems, for example, the motherboard and daughtercards comprise PCBs which may be multi-layer circuit boards having interconnect layers separated by and sandwiched between insulating layers. Digital signals and some analog signals not particularly sensitive to electromagnetic interference (EMI) are typically transmitted throughout the communication system over etch lines on the interconnect layers. Radio Frequency (RF) and Intermediate Frequency (IF) signals (operating in the range of 20-45 MHz) are usually transmitted over shielded wires such as coax or twin-ax to electronic circuitry on the PCBs, for example. The shielded RF cables, wires and connectors tend to be large and quite heavy, which is particularly a problem in the space and aircraft communications industry.
What is needed in the art is a PCB capable of transmitting RF/IF signals via etch lines embedded within the PCB, rather than over shielded RF cables and connectors of the prior art that are heavy and consume board space.
The present invention achieves technical advantages as a PCB having imbedded conductors for transmitting highly sensitive RF, IF and analog signals. A broadside coupled stripline is located in an RF circuit portion of the PCB being RF isolated from digital circuit portions. The broadside coupled stripline comprises a first conductor separated from a second conductor by a dielectric layer. The first and second conductors may conduct a differential signal, or one of the conductors may conduct an RF signal while the other is grounded and serves as a shield. The RF/IF, analog and digital portions of the PCB are separated and isolated from one another by use of a continuous ground shield constructed with via holes/pads and traces that form a layer by layer wall around the RF/IF, and possibly analog circuit portions. The remainder of the PCB may be utilized for digital circuits and power distribution.
In one embodiment, disclosed is a PCB having an embedded shielded conductor structure. The shielded conductor structure includes a first conductor, a dielectric layer disposed over the first conductor, and a second conductor proximate and juxtaposed to the first conductor and disposed over the dielectric layer, so that the first and second conductors form a broadside coupled stripline.
Also disclosed is a PCB with an embedded shielded conductor structure where the shielded conductor structure includes a first low noise ground layer positioned horizontally and a first conductor disposed in, a first conductive layer over the first low noise ground layer. A dielectric layer is disposed over the first conductor, and a second conductor is disposed in a second conductive layer over the dielectric layer proximate the first conductor. A second low noise ground layer is disposed over the second conductor, and a ground structure extends vertically through the various layers proximate the first and second conductors. The first and second conductors form a broadside coupled stripline, while the low noise ground layers and vertical ground structure provide additional EMI shielding, the low noise ground layers preferably being coupled to chassis ground.
Further disclosed is a method of manufacturing a PCB having an embedded shielded conductor structure defined about RF circuitry adapted to route RF signals. The method includes the steps of disposing a first conductive layer over a first dielectric layer, etching the first conductive layer to form a first conductor, and disposing a second dielectric layer over the first conductor. A second conductive layer is disposed over the second dielectric layer, and the second conductive layer is etched to form a second conductor proximate and juxtaposed to the first conductor to form a broadside coupled stripline.
The present invention provides the ability to maintain signal integrity of the RF/IF signals in a PCB without the use of a shielded cable, and provides overall weight, volume and cost reduction for electronic systems through the elimination of RF switches, cables and connectors. Passive odd-mode broadside coupled stripline is used to replace RF cabling distribution due to its excellent noise immunity and satisfactory characteristic impedance tolerance. Sensitive RF/IF signals, analog signals, digital signals and power supply voltages may be routed in a single PCB or motherboard backplane through the highly integrated circuit technology disclosed herein. RF/IF, analog and digital conductive etch lines may be disposed in a coplanar fashion in a single manufacturing step onto each layer of a PCB. RF signal routing through a motherboard or backplane using a distribution point or passive divider allows a single receiver RF output to be distributed to multiple analog-to-digital (A/D) converter inputs, for example, without compromising signal integrity and without the use of an RF switch, splitter or other switching networks.